During the Alchemists' Bank event, you can save up bonus coins and purchase them at a discount. When you win in locations and solve puzzles, in addition to the usual reward, you will receive bonus coins that get saved up in the alchemist chipmunk's cauldron. Your unused time and moves also earn you bonus coins. Once the chipmunk has a certain amount of coins saved up in the cauldron, you can buy them at a bargain price.

You can buy the coins as soon as the cauldron reaches the minimum required amount, or you can continue saving them until you reach the maximum amount.

The event is available for a certain period of time. You can find the timer under the event icon on the right side of the screen. Once you purchase the coins you've saved up, the event becomes unavailable. If you saved up coins for purchase but didn't buy them, you will have to start saving up coins from scratch the next time the event comes around.